Lens assembly with integrated feedback loop for focus adjustment

ABSTRACT

This invention provides a removably mountable lens assembly for a vision system camera that includes an integral auto-focusing liquid lens unit, in which the lens unit compensates for focus variations by employing a feedback control circuit that is integrated into the body of the lens assembly. The feedback control circuit receives motion information related to the bobbin of the lens from a position sensor (e.g. a Hall sensor) and uses this information internally to correct for motion variations that deviate from the lens setting position at a desired lens focal distance setting. Illustratively, the feedback circuit can be interconnected with one or more temperature sensors that adjust the lens setting position for a particular temperature value. In addition, the feedback circuit can communicate with an accelerometer that reads a direction of gravity and thereby corrects for potential sag in the lens membrane based upon the spatial orientation of the lens.

RELATED APPLICATION

This application is a divisional of co-pending U.S. patent applicationSer. No. 13/800,055, entitled LENS ASSEMBLY WITH INTEGRATED FEEDBACKLOOP FOR FOCUS ADJUSTMENT, filed Mar. 13, 2013, the teachings of whichare incorporated herein by reference.

FIELD OF THE INVENTION

This application relates to cameras used in machine vision and moreparticularly to automatic focusing lens assemblies.

BACKGROUND OF THE INVENTION

Vision systems that perform measurement, inspection, alignment ofobjects and/or decoding of symbology (e.g. bar codes, or more simply“IDs”) are used in a wide range of applications and industries. Thesesystems are based around the use of an image sensor, which acquiresimages (typically grayscale or color, and in one, two or threedimensions) of the subject or object, and processes these acquiredimages using an on-board or interconnected vision system processor. Theprocessor generally includes both processing hardware and non-transitorycomputer-readable program instructions that perform one or more visionsystem processes to generate a desired output based upon the image'sprocessed information. This image information is typically providedwithin an array of image pixels each having various colors and/orintensities. In the example of an ID reader, the user or automatedprocess acquires an image of an object that is believed to contain oneor more IDs. The image is processed to identify ID features, which arethen decoded by a decoding process and/or processor to obtain theinherent information (e.g. alphanumeric data) that is encoded in thepattern of the ID.

Often, a vision system camera includes an internal processor and othercomponents that allow it to act as a standalone unit, providing adesired output data (e.g. decoded symbol information) to a downstreamprocess, such as an inventory tracking computer system or logisticsapplication. It is often desirable that the camera assembly contain alens mount, such as the commonly used C-mount, that is capable ofreceiving a variety of lens configurations. In this manner, the cameraassembly can be adapted to the specific vision system task. The choiceof lens configuration can be driven by a variety of factors, such aslighting/illumination, field of view, focal distance, relative angle ofthe camera axis and imaged surface, and the fineness of details on theimaged surface. In addition, the cost of the lens and/or the availablespace for mounting the vision system can also drive the choice of lens.

An exemplary lens configuration that can be desirable in certain visionsystem applications is the automatic focusing (auto-focus) assembly. Byway of example, an auto-focus lens can be facilitated by a so-calledliquid lens assembly. One form of liquid lens uses two iso-densityliquids—oil is an insulator while water is a conductor. The variation ofvoltage passed through the lens by surrounding circuitry leads to achange of curvature of the liquid-liquid interface, which in turn leadsto a change of the focal length of the lens. Some significant advantagesin the use of a liquid lens are the lens' ruggedness (it is free ofmechanical moving parts), its fast response times, its relatively goodoptical quality, and its low power consumption and size. The use of aliquid lens can desirably simplify installation, setup and maintenanceof the vision system by eliminating the need to manually touch the lens.Relative to other auto-focus mechanisms, the liquid lens has extremelyfast response times. It is also ideal for applications with readingdistances that change from object-to-object (surface-to-surface) orduring the changeover from the reading of one object to anotherobject—for example in scanning a moving conveyor containing differingsized/height objects (such as shipping boxes). In general, the abilityto quickly focus “on the fly” is desirable in many vision systemapplications.

A recent development in liquid lens technology is available fromOptotune AG of Switzerland. This lens utilizes a movable membranecovering a liquid reservoir to vary its focal distance. A bobbin exertspressure to alter the shape of the membrane and thereby vary the lensfocus. The bobbin is moved by varying the input current within a presetrange. Differing current levels provide differing focal distances forthe liquid lens. This lens advantageously provides a larger aperture(e.g. 6 to 10 millimeters) than competing designs (e.g. Varioptic ofFrance) and operates faster. However, due to thermal drift and otherfactors, there may be variation in calibration and focus setting duringruntime use, and over time in general. A variety of systems can beprovided to compensate and/or correct for focus variation and otherfactors. However, these can require processing time (within the camera'sinternal processor) that slows the lens' overall response time in comingto a new focus. It is recognized generally that a control frequency ofat least approximately 1000 Hz may be required to adequately control thefocus of the lens and maintain it within desired ranges. This poses aburden to the vision system's processor, which can be based on a DSP orsimilar architecture. That is vision system tasks would suffer if theDSP were continually preoccupied with lens-control tasks.

SUMMARY OF THE INVENTION

This invention overcomes disadvantages of the prior art by providing aremovably mountable lens assembly for a vision system camera thatincludes an integral auto-focusing, liquid lens unit, in which the lensunit compensates for focus variations by employing a feedback controlcircuit that is integrated into the body of the lens assembly. Thefeedback control circuit receives motion information related to andactuator, such as a bobbin (which variably biases the membrane undercurrent control) of the lens from a position sensor (e.g., a Hallsensor) and uses this information internally to correct for motionvariations that deviate from the lens setting position at a target lensfocal distance setting. The defined “position sensor” can be a single(e.g. Hall sensor) unit or a combination of discrete sensors locatedvariously with respect to the actuator/bobbin to measure movement atvarious locations around the lens unit. Illustratively, the feedbackcircuit can be interconnected with one or more temperature sensors thatadjust the lens setting position for a particular temperature value. Inaddition, the feedback circuit can communicate with an accelerometerthat senses the acting direction of gravity, and thereby corrects forpotential sag (or other orientation-induced deformation) in the lensmembrane based upon the spatial orientation of the lens.

In an illustrative embodiment, a lens assembly for a vision systemcamera having variable focus provides a lens body having a variable lensassembly and a fixed optics assembly. A controller (control circuit) islocated within the body. The controller is constructed and arranged toreceive a target focal distance from the vision system camera. Thecontroller generates a target position of an actuator that controlscurvature of the variable lens assembly. Based upon an actual measuredposition of the actuator, the controller corrects the measured positionof the actuator to the target continuously, in a feedback loop.Illustratively, the variable lens assembly includes a membrane-basedliquid lens element in which the membrane curvature is driven by amoving actuator. The liquid lens element can include a position sensorlocated to measure movement of the actuator associated with movement ofa membrane of the membrane-based liquid lens assembly. This positionsensor can comprise one or more linear Hall sensor(s) that measure(s) amagnet positioned to move on the actuator. The actuator can be a bobbinthat is driven by current using a current controller operativelyconnected with the controller. The target position informationillustratively defines a position that focuses an image acquired by thevision system camera. Additionally, the target position information canbe further corrected by the controller for at least one of temperatureof the liquid lens assembly, spatial orientation and/or other parameters(e.g. flange-to-sensor distance tolerance) of the liquid lens assembly.Thus, the controller converts this information into a corrected targetposition value for the Hall sensor. The corrected position informationis determined by the controller based upon stored calibration parametersthat reside in the memory (e.g. an EEPROM of the lens assembly). Thecalibration parameters can relate to temperature of the lens, providedby a temperature sensor, spatial orientation of the lens, provided by anaccelerometer, and/or other parameters, such as flange-to-sensordistance tolerance. The controller can also allow for upgrade of itsprocess instructions (firmware) via the communication network (e.g. anI2C communication interface), typically upon startup. This firmwareupgrade is received from the vision system if newer information isavailable from it.

Illustratively, the controller can reside on a circuit board that ispositioned on a shelf surrounded by a cap assembly. The (e.g.,cylindrical) cap assembly surrounds a filler having the shelf and a mainbarrel assembly that contains the fixed optics therein. The cap assemblyis operatively connected to the filler containing the shelf. It isselectively rotatable about an optical axis with respect to the mainbarrel assembly. The main barrel assembly includes a mount baseconstructed and arranged to removably secure to a mount of the visionsystem camera so that the lens assembly is exchangeable. The controllerillustratively indicates when the lens position has moved to a correctedposition.

In an illustrative embodiment, a method for controlling focus of amembrane-based liquid lens assembly of a vision system camera in theform of a “local” feedback loop (i.e. using a lens-assembly basedcontroller/processor includes measuring of a present position of anactuator of the membrane-based liquid lens assembly with a positionsensor. A target position of the actuator is received from aninterconnected vision system processor of the vision system camera inthe form of a focal distance. This distance is interpreted into thetarget position of the actuator by the controller. The controller(locally mounted in a body of the lens assembly) compares the measured,actual position of the actuator with the target position, and determineswhether the two positions are currently substantially equal. If thevalues are substantially equal, then a correct position is indicated bythe controller. If the values are sufficiently unequal, then thecontroller sends a correction to the actuator and repeats the abovesteps in a feedback loop that continuously maintains correct positionbased upon the current target.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a perspective view of the external structure of anexchangeable auto-focus lens assembly with integratedfeedback-loop-based focus control according to an illustrativeembodiment;

FIG. 2 is a side cross section of the lens assembly of FIG. 1 showingthe layout of internal mechanical, optical, electro-optical andelectronic components;

FIG. 3 is a perspective view of the lens assembly of FIG. 1 with outercap removed to reveal the arrangement of components;

FIG. 4 is a perspective view of the lens assembly of FIG. 1 with theouter cap and spacer assembly removed to reveal the interconnectionbetween the liquid lens and the control circuit;

FIG. 5 is a block diagram of the generalized electrical connection anddata flow between the liquid lens, integrated controller and cameravision system processor for the lens assembly of FIG. 1;

FIG. 5A is a flow diagram of a feedback loop-based bobbin positioncontrol process for the lens assembly of FIG. 1;

FIG. 6 is a block diagram of the stored data in the control circuitmemory of FIG. 5; and

FIG. 7 is a temperature correction process that generatestemperature-corrected bobbin position values for use with the controlcircuit of FIG. 5 and process of FIG. 5A.

DETAILED DESCRIPTION

FIG. 1 details the external structure of an exchangeable, auto-focuslens assembly (also simply termed “lens assembly”) 100 according to anillustrative embodiment. The lens assembly includes an outer cap 110defining a generally cylindrical shape. This outer cap 110 provides aprotective and supporting shell for a variable focus lens element(comprising an Optotune membrane-based liquid lens model EL-6-18 orEL-10-30 in this exemplary embodiment) 120. By way of useful backgroundinformation the present data sheet with specifications for variousmodels of this lens is available on the World Wide Web atwww.optotune.com/images/products/Optotune %20EL-6-18.pdf. It isexpressly contemplated that the teachings of the embodiments herein canbe applied to a variety of electronically focused lens types includingother forms of liquid lens technology and electro-mechanically adjustedsolid lenses. For the purposes of this description, the variable focuslens element 120 (also simply termed the “liquid lens”) of the overallauto-focus lens assembly 100 is assumed to operate based uponpredetermined inputs of current (or voltage in alternate arrangements),and provides various outputs that the user can employ to monitor andcontrol the lens using conventional techniques. Such outputs can includethe position of the bobbin using, for example, one or more Hall sensors(described further below) and/or the present temperature of the lensusing one or more conventional temperature sensors.

By way of further background, it has been observed that such liquidlenses exhibit excessive drift of its optical power over time andtemperature. Although the lens can be focused relatively quickly to anew focal position (i.e. within 5 milliseconds), it tends to drift fromthis focus almost immediately. The initial drift (or “lag”) is caused bylatency in the stretch of the membrane from one focus state to thenext—i.e. the stretch takes a certain amount of time to occur. A seconddrift effect with a longer time constant is caused by the powerdissipation of the lens' actuator bobbin heating up the lens membraneand liquid. In addition the orientation of the lens with respect to theacting direction of gravity can cause membrane sag that has an effect onfocus. The system and method of the embodiments described herein addressdisadvantages observed in the operation and performance such liquidlenses.

The rear 130 of the lens assembly 100 includes a threaded base that canbe adapted to seat in a standard camera mount, such as the popular cineor (C-mount). While not shown, it is expressly contemplated that thelens assembly 100 can be (removably) mounted a variety of camera typesadapted to perform vision system tasks with an associated vision systemprocessor.

With further reference also to FIGS. 2-4, the construction of the lensassembly 100 is described in further detail. It is expresslycontemplated that the depicted construction is illustrative of a rangeof possible arrangements of components that should be clear to those ofskill in the art. The cap 110 defines a metal shell (for examplealuminum alloy) that includes a side skirt 140 and unitary front face150. The cap overlies a spacer/filler 210 (see also FIG. 3). This filler210 includes a pair of threaded holes 310 (FIG. 3) that receive threadedfasteners 160 to removably secure the cap over the filler 210. A pair ofopposing threaded fasteners 170 are recessed in corresponding holes 172of the cap and pass through holes 320 in the filler 210 and intothreaded holes 410 (FIG. 4) on two keys 440 that rotatably engage themain lens barrel assembly 220 (FIGS. 2 and 4). This relationship isdescribed further below. These fasteners 170 maintain the main lensbarrel assembly 220 in axial alignment with the filler 210.

As shown in FIG. 2, the lens barrel assembly 220 contains a series offixed lenses 230, 232, 234, 236 and 238 arranged according to ordinaryoptical skill behind the liquid lens 210. These lenses allow the imageprojected along the optical axis OA to the vision system sensor to besized appropriately to the sensor's area over a range of varying focaldistances specified for the lens assembly. By way of example, the rangeof optical power can be −2 to +10 diopter. The lenses 230-238 arearranged in a compressed stack within the main barrel assembly 220 withappropriate steps and/or spacers therebetween. The overall stack is heldin place by a threaded retaining ring 240 at the rear end (130) of thelens assembly 110. At the front of the main barrel is located anaperture stop disc 250 that reduces the system aperture to anappropriate, smaller diameter. This allows customization ofbrightness/exposure control and/or depth of field for a given visionsystem application.

The main barrel assembly 220 includes a rear externally threaded base260 having a diameter and thread smaller than that of a C-mount—forexample a conventional M-12 mount size for interchangeability withcamera's employing this standard, or another arbitrary thread size. Athreaded mount ring 262 with, for example, a C-mount external thread 264is threaded over the base thread 260. This ring 262 allows the backfocus of the lens with respect to the camera sensor to be accuratelyset. In general, the shoulder 266 of the ring is set to abut the face ofthe camera mount when the lens is secured against the camera body. Apair of set screws 360 (FIGS. 3 and 4) pass through the ring 262, andremovably engage the base thread 260 to maintain the mount ring 262 atan appropriate back focus setting.

An O-ring 267 is provided on the front face of the liquid lens 120 tocancel out tolerances. In addition, and with reference also to FIG. 4,filler 210 is adapted to rotate with respect to the main barrel assembly220. A pair of semi-circular keys 440, held together by an O-ring 450engage a groove in the filler 210 and allow the filler 210 and cap 110to rotate with respect to the barrel assembly 220 about the axis OA,while fixing these components along the axial direction. In this manner,after the lens assembly threaded base is properly seated in the camerahousing with desired back focus, the cap is rotated to align the cable270 with the camera's connecting socket. This rotation is secured viathe knob 180 (FIG. 1) that threads through a hole 380 in the filler 210and can be tightened to bear against the barrel assembly 220, therebyrotationally locking these components together at the desired rotationalorientation therebetween.

As shown in FIG. 3, the front end of the filler 210 includes a somewhatrectangular recess 330 to support the shape of the liquid lens 120 in aposition at the front of the assembly and in front of the main barrelassembly 220. The filler 210 also includes a flattened top end (shelf)340 with appropriate raised retaining tabs 342 to support a lens controlcircuit board 350 according to an illustrative embodiment. Thearrangement of the shelf 340, circuit board 350 and cap 110 define asufficient gap G (FIG. 2) between the inner surface of the cap and thecircuit board to provide clearance for the board. In an embodiment, theapproximate diameter of the cap is approximately 32 millimeters.

Notably, the barrel assembly 220 is an interchangeable component so thatdifferent fixed lens arrangements can be provided in the overall lensassembly (i.e. with the same liquid lens, cap and control circuitry).Thus, this design provides substantial versatility in providing a rangeof possible focal distances for different vision system applications.

Also notably, the provision of a lens control circuit within the overallstructure of the lens assembly allows certain control functions to belocalized within the lens itself. This is described in further detailbelow. The circuit board 350 is connected via a connector 422 andstandard ribbon cable 420 to the liquid lens 120 as shown in FIG. 4. Thefiller 210 provides a gap to run the cable 420 between these components.Additionally, the control circuit board 350 is connected to a cable 270and multi-pin end connector 272. These are arranged to electricallyconnect to a receptacle on the camera housing (typically along its frontface adjacent to the lens mount). This cable provides power to the lensassembly (the circuit board and liquid lens) from the camera body, andalso provides a data interconnect between the lens and the camera'svision system processor, as described in further detail below. A cutout274 at the rear edge of the cap 110 provides a chase for the cable 270to pass from the interior to the exterior of the lens assembly 110.Appropriate seals and/or close-tolerance fits prevent incursion ofmoisture or contaminants from the environment.

The control functions of the circuit board 350 are now described infurther detail with reference to FIG. 5. As described above, it has beenobserved that the drift or lag can be controlled by measuring theposition of the actuator and the temperature of the lens and using thisdata to control the current through the lens actuator bobbin (a magneticcoil that compresses the lens variable under different currentsettings). In an illustrative embodiment, such drift/lag is compensatedby a control circuit 510 (also termed simply “controller”) on thecircuit board that integrates a (digital) feedback loop completely intothe lens barrel of the lens assembly avoiding the use of the camera'svision system processor to control these adjustments. The controlcircuit includes an associated memory (e.g. an EEPROM) 512 that, asshown in FIG. 6 can be divided into data memory 610 and program memory620. As described further below, the data memory 610 can includecorrection parameters for temperature 612, orientation with respect togravity 614, and other appropriate parameters 616. Such other parameters616 can include tolerance control parameters, such as the flangetolerance correction (described below). The program memory can includethe feedback-loop control software and correction application 622.

At startup, the vision system 520 communicates to the lens assemblycircuit 350 the tolerance value of its flange-to-sensor distance. Thisvalue is the deviation from the ideal C-mount distance (typically 17.526millimeters), which has been measured after assembly of the visionsystem and has been stored in the memory 526 (e.g. a non-volatile flashmemory) of the vision system. The control circuit 510 is arranged tocorrect for the flange tolerance as described further below.

Upon startup, the control circuit 510 can request the vision systemprocessor 522 of the vision system camera 520 to provide the latestfirmware upgrade 528 so that the function lens assembly is synchronizedwith the software and firmware of the vision system. If the firmware isup-to-date, then the processor indicates this state to the lens controlcircuit and no upgrade is performed. If the firmware is out-of-date,then the new firmware is loaded in the appropriate location of the lensassembly's program memory 620 (FIG. 6). This communication typicallyoccurs over the lens assembly's I2C communication interface (531)transmitted over the cable 270 (FIG. 2).

Note, as used herein the terms “process” and/or “processor” should betaken broadly to include a variety of electronic hardware and/orsoftware based functions and components. Moreover, a depicted process orprocessor can be combined with other processes and/or processors ordivided into various sub-processes or processors. Such sub-processesand/or sub-processors can be variously combined according to embodimentsherein. Likewise, it is expressly contemplated that any function,process and/or processor herein can be implemented using electronichardware, software consisting of a non-transitory computer-readablemedium of program instructions, or a combination of hardware andsoftware.

The control circuit 510 can be implemented using a variety of electronichardware. Illustratively a microcontroller is employed. The controlcircuit 510 receives focus information 530 (e.g. focal distance, whichis translated by the controller into target bobbin position) from thevision system camera 520 (i.e. via cable 270 and interface link 531).This focus information can be derived from a focus process 532 thatoperates in the camera processor 522. The focus process can useconventional or custom auto-focus techniques to determine proper focus.These can include range-finding or stepping through a series of focusvalues in an effort to generate crisp edges in the image 534 of anobject acquired by the sensor 536. While highly variable a 2K×1K-pixelsensor is used in the exemplary embodiment.

The focus information 530 is used by the control circuit 510 to generatea target bobbin position and to provide a digital signal with movementinformation 540 to the current controller 544. The current controllerapplies the appropriate current to an annular bobbin assembly 550 (or“bobbin”), which thereby deforms the liquid lens membrane 552 to providean appropriate convex shape to the bulged lensmatic region 554 withinthe central opening of the bobbin 550. The bobbin 550 includes a magnet558 that passes over a conventional linear Hall sensor 560. This Hallsensor 560 generates a digital position signal 562 that is directed backto the control circuit 510 where it is analyzed for actual bobbinposition (for example, calling up values in the memory 512) versus thetarget position represented by a corresponding Hall sensor targetposition. If, in a comparison of the actual Hall sensor value and targetHall sensor value, these values do not match, then the control circuit510 applies a correction, and that is delivered to the currentcontroller 544, where it is used to move the bobbin 550 to a correctposition that conforms with the target Hall sensor position. Once thebobbin 550 is at the correct position, the controller can signal thatcorrection is complete.

Note that additional Hall sensors (or other position-sensing devices)566 (shown in phantom) can generate additional (optional) positionsignals 568 that are used by the control circuit to verify and/orsupplement the signal of sensor 560. In an embodiment, data istransmitted between components using an I2C protocol, but otherprotocols are expressly contemplated. In general, the commerciallyavailable Hall sensor operates in the digital realm (i.e. using the I2Cinterface protocol), thereby effectively avoiding signal interferencedue to magnetic effects. By way of non-limiting example, a model AS5510Hall linear sensor (or sensors) available from AustriaMicrosystems (AMS)of Austria can be used.

With reference to FIG. 5A, a bobbin position-sensing/correcting feedbackloop process 570 is shown in a series of flow-diagram process steps. Atarget focus distance is received from the vision system processor instep 572. The control feedback loop 570 then initiates as this focusdistance is used by the lens assembly control circuit (controller) 510to determine a target value for bobbin position represented by a targetHall sensor value provided by one or more sensors on the bobbin. Thetarget Hall sensor value(s) can be corrected based upon storedparameters in memory 512 (step 574). Such parameters include, but arenot limited to temperature, spatial orientation andflange-to-sensor-distance tolerance, and this (optional) process isdescribed further below. In step 576, the control circuit 510 measuresthe actual position of the bobbin based upon the position of the Hallsensor(s) and associated signal value(s) (562). In step 578, the controlcircuit 510 then compares the actual, returned Hall sensor value(s) withthe target value. If the values are not substantially equal thendecision step 580 branches to step 582 and the control circuit directsthe current controller 544 to input a current that will move the bobbinto the corrected position. This can be based on the difference incurrent needed to move the bobbin between the actual and correctposition. If the comparison in step 578 determines that the actual andtarget Hall sensor value(s) are substantially equal, then the decisionstep 580 branches to step 582 and the system indicates that correctionis complete. The control circuit repeats correction steps 574, 576, 578,580 and 582 until the actual and target Hall sensor values aresubstantially equal (within an acceptable tolerance), and the newcorrect bobbin position is indicated. This complete status can bereported to the camera's processor 522 for use in performing imageacquisition.

Note that this local feedback loop 570 can run continuously to maintainfocus at a set position once established, and until a new bobbinposition/focus is directed by the camera. Thus, the feedback loop 570ensures a steady and continuing focus throughout the image acquisitionof an object, and does so in a manner that avoids increased burdens onthe camera's vision system processor.

The determination of the target value for the Hall sensor(s) in step 574can include optional temperature, spatial orientation and/or otherparameter (e.g. flange distance) correction based upon parameters 612,614, 616 (FIG. 6) stored in memory 512. Temperature of the lens unit issensed (optionally) by an on-board or adjacent temperature sensor 588(FIG. 5). The temperature sensor 588, like other components of thecircuit 350, can employ a standard interface protocol (e.g. I2C).

As shown in FIG. 7, an optional temperature compensation process 700operating within the control circuit 510 receives a temperature reading710 from the sensor 536 and target focus or bobbin position information720 and applies temperature calibration parameters 730. These can bestored locally on the lens assembly circuit memory 512 as shown in FIG.6. The correction parameters can define a curve or a series of tablevalues associated with given temperature readings that are measuredduring calibration. The process 700 modifies the target Hall sensorvalue (and associated bobbin position) from a base value, based upon thefocus distance provided by the vision system camera to a value thataccounts for the variation of lens focus with respect to lenstemperature. Thus, the base Hall sensor value can be added-to orsubtracted from by the control circuit 510 based upon the prevailingtemperature reading at the lens to generate a temperature correctedtarget value 740.

Likewise, correction for orientation with respect to gravity that canresult in sag or other geometric deformation of the lens membrane indiffering ways is compensated by an (optional) accelerometer 594 thattransmits the spatial orientation 596 of the lens/camera with respect tothe acting directing of gravity to the control circuit via, for example,an I2C protocol. In an embodiment, an orientation correction factor isdetermined (by reading the accelerometer 594), and applied to the targetHall sensor value by the control circuit in a manner similar totemperature correction (FIG. 7) substituting orientation for temperaturein block 710. Since orientation typically remains constant (except inthe case of a moving camera), the determination of orientation can be aone-time event (i.e. at camera setup/calibration), or can occur uponstart up or at a timed interval based upon the control circuit's clock.Like temperature correction, orientation correction parameters cancomprise a curve or lookup table mapped to differing orientations, whichcan be determined during calibration. The appropriate orientationparameter value is applied to the step of determining (574) the targetHall sensor value, and the target value is adjusted to include thisfurther correction for spatial orientation. Note that in the case of amoving camera, the orientation parameter can be continuously updated inthe same manner that temperature is updated to account for changes overtime.

Other parameters (616 in FIG. 6), such as flange-to-sensor distancetolerance, can also be stored in the circuit memory 512. Theseparameters can be updated from the data store of the vision systemcamera upon startup or at another interval of time. The value of eachparameter is used by the control circuit's process to further adjust andcorrect the target Hall sensor value. This overall corrected value isused in the comparison step 578 against the actual measured value tothereby move the bobbin to the correct position.

It should be clear that superior position correction, on the order of 1millisecond, can be achieved using the local feedback loop instantiatedin a control circuit packaged in the lens assembly. The entire lensassembly package fits within a standard C-mount lens affording a highdegree of interoperability with a wide range of vision system cameramodels and types. The system and method for controlling and correctingthe focus of a liquid (or other similar auto-focusing) lens describedherein can be employed rapidly, and at any time during camera runtimeoperation and generally free of burden to the camera's vision systemprocessor. This system and method also desirably accounts for variationsin focus due to thermal conditions and spatial orientation (i.e. lenssag due to gravity). This system and method more generally allow for alens assembly that mounts in a conventional camera base.

The foregoing has been a detailed description of illustrativeembodiments of the invention. Various modifications and additions can bemade without departing from the spirit and scope of this invention.Features of each of the various embodiments described above can becombined with features of other described embodiments as appropriate inorder to provide a multiplicity of feature combinations in associatednew embodiments. Furthermore, while the foregoing describes a number ofseparate embodiments of the apparatus and method of the presentinvention, what has been described herein is merely illustrative of theapplication of the principles of the present invention. For example,while a Hall sensor is used to measure position, a variety of alternateposition-sensing devices can be used in association with the feedbackloop herein. For example an optical/interference-based position sensorcan be employed in alternate embodiments. Also, it is contemplated thatthe principles herein can be applied to a variety of lenses (liquid andotherwise), in which the curvature of the lens is varied via electroniccontrol. Thus the term “variable lens assembly” should be taken broadlyto expressly include at least such lens types. In addition while variousbobbin position corrections are performed within the lens controlcircuit and feedback loop, it is contemplated that some corrections canbe performed within the vision system camera processor, and thecorrected focal distance is then sent to the lens assembly for use infurther feedback loop operations. As used herein, various directionaland orientation terms such as “vertical”, “horizontal”, “up”, “down”,“bottom”, “top”, “side”, “front”, “rear”, “left”, “right”, and the like,are used only as relative conventions and not as absolute orientationswith respect to a fixed coordinate system, such as gravity. Accordingly,this description is meant to be taken only by way of example, and not tootherwise limit the scope of this invention.

What is claimed is:
 1. A method for controlling focus of amembrane-based liquid lens assembly of a vision system camera comprisingthe steps of: reading, with a local controller mounted in a lens bodycontaining the liquid lens assembly, a target focal position receivedfrom a vision system processor of a vision system camera anddetermining, based on the target focal position, a target position of anactuator of the membrane-based liquid lens assembly; measuring, basedupon a position sensor associated with movement of the actuator, anactual position of the actuator; comparing the measured actual positionwith the target position, and determining a corrected position that isassociated with the target position; and instructing movement of aposition of the actuator to the corrected position.
 2. The method as setforth in claim 1 further comprising signaling when the actuator hassuccessfully moved to the corrected position.
 3. The method as set forthin claim 1, further comprising, correcting the target position basedupon at least one of (a) a measured temperature of the liquid lensassembly, (b) a spatial orientation of the liquid lens assembly relativeto an acting direction of gravity and (c) a stored sensor-to-flangedistance tolerance associated with a mount of the lens body.
 4. Themethod as set forth in claim 1 wherein the step of measuring the actualposition comprises receiving a measurement value from at least one Hallsensor.
 5. The method as set forth in claim 1 wherein the step ofinstructing movement comprises varying current to a bobbin based upon acontroller.
 6. The method as set forth in claim 1 wherein the step ofdetermining the target position comprises determining a position thatfocuses an image acquired by the vision system camera.
 7. The method asset forth in claim 6 wherein the target position defines positioninformation that is further corrected for at least one of temperature ofthe liquid lens assembly, spatial orientation of the liquid lensassembly and flange-to-sensor distance tolerance.
 8. The method as setforth in claim 1 wherein the step of determining the corrected positionis based upon accessing calibration parameters stored in a memoryoperatively connected to the controller and located in the lensassembly.
 9. The method as set forth in claim 1, further comprising,receiving, from the vision system camera, a firmware update for storagein a memory operatively connected to the controller and located in thelens assembly when a current firmware version in the memory isout-of-date compared to a version stored by the vision system camera.10. The method as set forth in claim 1, further comprising, indicatingwhen the lens position has moved to a corrected position.
 11. A methodfor controlling focus of a membrane-based liquid lens assembly of avision system camera comprising the steps of: receiving, at a localcontroller mounted in a lens body containing the liquid lens assembly, atarget focal distance provided by a vision system processor of thevision system camera; determining, based on the target focal distance, atarget position of an actuator of the membrane-based liquid lensassembly; determining, based upon a position sensor associated withmovement of the actuator, a current position of the actuator; comparingthe current position with the target position, and determining acorrected position that is associated with the target position; andinstructing movement of a position of the actuator to the correctedposition.
 12. The method as set forth in claim 11, further comprisingmoving a membrane of the liquid lens assembly via movement of theposition of the actuator.
 13. The method as set forth in claim 11,further comprising, correcting the target position based upon at leastone of (a) a measured temperature of the liquid lens assembly, (b) aspatial orientation of the liquid lens assembly relative to an actingdirection of gravity and (c) a stored sensor-to-flange distancetolerance associated with a mount of the lens body.
 14. The method asset forth in claim 11 wherein the step of determining the currentposition comprises receiving a measurement value from a Hall sensor. 15.The method as set forth in claim 11 wherein the step of instructingmovement comprises varying a current supplied to the actuator.
 16. Themethod as set forth in claim 11 wherein the step of instructing movementcomprises supplying a current to an annular bobbin assembly of theactuator.
 17. The method as set forth in claim 11 further comprisingreceiving a temperature of the liquid lens assembly, and determining thecorrected position using the received temperature.
 18. The method as setforth in claim 11 further comprising receiving a spatial orientation ofthe liquid lens assembly, and determining the corrected position usingthe received spacial orientation.
 19. The method as set forth in claim18, wherein the spatial orientation is received from an accelerometerassociated with the vision system camera.